Two advanced styrene-butadiene/polybutadiene rubber blends filled with a silanized silica nanofiller for potential use in passenger car tire tread compound

Two Advanced Styrene-Butadiene/Polybutadiene RubberBlends Filled with a Silanized Silica Nanofiller forPotential Use in Passenger Car Tire Tread Compound

Farhan Saeed,1 Ali Ansarifar,1 Robert J. Ellis,2 Yared Haile-Meskel,2 M. Shafiq Irfan1

1Department of Materials, Loughborough University, Leicestershire, LE11 3TU, United Kingdom2DTR VMS Ltd, Bumpers Way, Chippenham, Wiltshire SN14 6NF, United Kingdom

Received 9 November 2010; accepted 26 January 2011DOI 10.1002/app.34221Published online 17 August 2011 in Wiley Online Library (wileyonlinelibrary.com).

ABSTRACT: Styrene-butadiene rubber (SBR) and poly-butadiene rubber (BR) were mixed together (75:25 bymass) to produce two SBR/BR blends. The blends were re-inforced with a precipitated amorphous white silica nano-filler the surfaces of which were pretreated with bis(3-triethoxysilylpropyl)-tetrasulfide (TESPT). TESPT is a sul-fur-bearing bifunctional organosilane that chemicallybonds silica to rubber. The rubbers were primarily curedby using sulfur in TESPT and the cure was optimized byadding non-sulfur donor and sulfur donor acceleratorsand zinc oxide. The hardness, Young’s modulus, modulusat different strain amplitudes, tensile strength, elongationat break, stored energy density at break, tear strength,cyclic fatigue life, heat build-up, abrasion resistance, glasstransition temperature, bound rubber and tan d of thecured blends were measured. The blend which was cured

with the non-sulfur donor accelerator and zinc oxide hadsuperior tensile strength, elongation at break, storedenergy density at break and modulus at different strainamplitudes. It also possessed a lower heat build-up, ahigher abrasion resistance and a higher tan d at low tem-peratures to obtain high-skid resistance and ice and wet-grip. Optimizing the chemical bonding between the rubberand filler reduced the amount of the chemical curatives byapproximately 58% by weight for passenger car tire tread.This helped to improve health and safety at work andreduce damage to the environment. VC 2011 Wiley Periodicals,Inc. J Appl Polym Sci 123: 1518–1529, 2012

Key words: rubber; blends; silicas; crosslinking; mechanicalproperties

INTRODUCTION

The invention of wheel almost 55,000 years ago hasbeen one of the greatest achievements of mankind.The wheel is used on cars, planes and farm equip-ment. The first air filled, pneumatic tire waspatented in 1888 with a specific aim to replace hardrubber tires and provide a smooth and high speedride for cyclists. Eventually, car tires were inventedbut the early ones lasted up to 40 miles on averagebefore repair or replacement was needed. Indeed,car tires have come a long way and this was mainlydue to the brilliance of rubber scientists and tech-nologists who used chemical ingredients such asprocessing aids, accelerators, activators, fillers, andantidegradants to improve the processing andmechanical properties of rubber compounds for tireapplications. A great deal is also owed to tire manu-

facturers who developed more efficient and refinedtire testing procedures and advanced technology.To improve the mechanical properties of a rubber

for example hardness, tear strength, tensile strengthand elongation at break, fillers with surface areasfrom 150 to 400 m2/g such as colloidal carbonblacks,1,2 synthetic silicas,3–4 organoclays,5 and metaloxides,6 are added. In addition to filler, compoundsused to manufacture industrial rubber articles suchas passenger car tire tread contain up to eight classesof rubber chemicals. For instance, the cure systemconsists of up to five different chemicals; primaryand secondary accelerators, primary and secondaryactivators and elemental sulfur, which may add upto 11 parts per hundred rubber by weight (phr).7

Antidegradants and processing aids are alsoincluded to protect rubber against environmentalageing and to improve processing properties,respectively.Fillers and curing chemicals perform two distinct

functions in rubber compounds. Fillers increase themechanical properties2,4,8,9 and curing chemicalsproduce crosslinks between the rubber chains at ele-vated temperatures, i.e., 140–240�C.10,11 Syntheticprecipitated amorphous white silica filler is replac-ing carbon blacks in some applications such as tiretread compound. This filler has silanol or hydroxyl

Correspondence to: A. Ansarifar ([email protected]).

Contract grant sponsor: DTR VMS Limited of UK(formerly Avon Automotive VMS).

Journal of Applied Polymer Science, Vol. 123, 1518–1529 (2012)VC 2011 Wiley Periodicals, Inc.

groups on its surfaces which make it acidic12 andmoisture adsorbing.13 Acidity and moisture are bothdetrimental to the cure of rubber compounds14 andcan also cause loss of crosslink density in sulfur-cured rubbers.4 Furthermore, when a large amountof silica is added, processing becomes more difficultbecause the rubber viscosity increases significantly.15

Bifunctional organosilanes such as bis(3-triethoxysi-lylpropyl)-tetrasulfide (TESPT) are used to enhancethe reinforcing capability of fillers with silanolgroups on their surfaces, such as precipitated silicas,and also forms an integral part of curing systems toimprove the crosslinking network properties.14

TESPT combines silica and sulfur into one singleproduct known as a ‘‘crosslinking filler.’’16 One suchfiller is silanized silica where the surfaces of precipi-tated silica are pretreated with TESPT to chemicallybond silica to rubber (Scheme 1) and to prevent thefiller from interfering with the reaction mechanismof sulfur-cure in rubber.4.

Using a precipitated silica filler pretreated withTESPT, a substantial reduction in the use of the cur-ing chemicals was achieved in natural rubber(NR),17 styrene-butadiene rubber (SBR),18 polybuta-diene rubber (BR),19 and nitrile rubber (NBR)20 with-out compromising the good mechanical properties ofthe rubber vulcanizates. To crosslink the rubber andoptimize the chemical bonding between the rubberand filler via TESPT, accelerators and activatorswere added. The mechanical properties of the curedrubbers were improved significantly in spite ofreducing the amount of the curing chemicals.

The aim of this study was to develop two styrene-butadiene/polybutadiene rubber blends (75 : 25 bymass) for potential use in passenger car tire treadformulations. The mass fraction of SBR with respectto BR in some passenger tire-tread compounds is 75: 25. The blends were reinforced and crosslinkedwith a high loading of a silanized silica nanofiller,and the chemical bonding between the rubber andfiller was optimized by adding sulfenamide and

thiuram accelerators and zinc oxide activator. Thehardness, tensile strength, elongation at break,stored energy density at break, tear strength,Young’s modulus, modulus at different strain ampli-tudes, bound rubber, cyclic fatigue life, heat build-up, abrasion resistance, glass transition temperature,and tan d of the blends were measured.

EXPERIMENTAL

Materials: Rubber and nanofiller

The raw elastomers used were styrene-butadienerubber (SBR) (23.5 wt % styrene; Intol 1712, PolimeriEuropa UK Ltd., Hythe, UK) and high-cis polybuta-diene rubber BR (96% 1,4-cis; Buna CB 24, Bayer,Newbury, UK; not oil extended). SBR Intol 1712 is acold emulsion copolymer, polymerized using a mix-ture of fatty acid and rosin acid soaps as emulsifiers.It is extended with 37.5 phr of highly aromatic oiland contains a styrenated phenol as a non-stainingantioxidant. It has approximately 4.8% by weight or-ganic acid. The reinforcing filler was Coupsil 8113,which was supplied by Evonik Industries AG ofGermany. Coupsil 8113 is a precipitated amorphouswhite silica-type Ultrasil VN3 surfaces of which hadbeen pretreated with TESPT. It has 11.3% by weightsilane, 2.5% by weight sulfur (included in TESPT), a175 m2/g surface area (measured by N2 adsorption)and a 20–54 nm particle size.

Curing chemicals, antidegradants,and processing oil

In addition to the raw rubbers and filler, the otheringredients were N-tert-butyl-2-benzothiazole sul-fenamide (a safe-processing delayed action nonsulfur-donor accelerator with a melting point of105�C) (Santocure TBBS, Flexsys, Dallas, TX), Tetra-methyl thiuram disulfide (a fast curing sulfur-donoraccelerator with �13% of the sulfur available to reactwith rubber and a melting point of 146�C) (PerkacitTMTD PDR D, Flexsys, Belgium, Europe), zinc oxide(ZnO; an activator, Harcros Durham Chemicals,Durham, UK), N-(1,3-dimethylbutyl)-N0-phenyl-p-phenylenediamine (an antidegradant, Santoflex 13,Brussels, Belgium), and heavy paraffinic distillatesolvent extract containing aromatic oils (a processingoil, Enerflex 74, Milton Keynes, UK). The oil wasadded to reduce the rubber viscosity and the antide-gradant to protect the rubber against environmentalageing. The cure system consisted of TBBS, TMTD,and ZnO, which were added to fully crosslink therubber.

Mixing equipment

The compounds were prepared in a Haake Rheocord90 (Berlin, Germany), a small size laboratory mixer

Scheme 1 Chemical structure of bis(3-triethoxysilyl-propyl)-tetrasulfide (TESPT).

ADVANCED PASSENGER CAR TIRE TREAD COMPOUNDS 1519

Journal of Applied Polymer Science DOI 10.1002/app

with counter rotating rotors. The Banbury rotors andthe mixing chamber were initially set at ambienttemperature (23�C) and the rotor speed was set at45 r.p.m. The volume of the mixing chamber was78 cm3, and it was 57% full during mixing. Polylabmonitor 4.17 software was used for controlling themixing condition and storing data.

Assessment of the silica dispersion in the rubber

To select a suitable mixing time for incorporatingthe silica in the rubbers, the mixing time wasincreased to 22 min to disperse the silica particlesfully in the rubber. Twenty-four hours after the mix-ing ended, the rubbers were examined in a scanningelectron microscope (SEM) to assess the filler disper-sion. Dispersion of the silica particles in the rubberwas assessed by a Cambridge Instruments Stereo-scan 360 Tungsten filament scanning electron micro-scope. Small pieces of the uncured rubber wereplaced in liquid nitrogen for 3 min and then frac-tured to create two fresh surfaces. The samples,65 mm2 in area and 7 mm thick, were coated withgold and then examined and photographed in theSEM. The degree of dispersion of the silica particlesin the rubber was then studied from SEM photo-graphs. For example, Figure 1 shows a typical gooddispersion of the silica particles in the rubber. Afterthe photos were examined, a total mixing time of10 min for the SBR rubber and 16 min for the BRrubber, respectively, were found to be sufficient tofully disperse the silica particles in the rubbers.These mixing times were subsequently used to makerubber compounds for this study.

Mixing procedure

To prepare the SBR compounds, TBBS, TMTD andZnO were added 4 min after the filler and rubber

were mixed together, and mixing continued subse-quently for another 6 min before the compoundswere removed from the mixer. The SBR compoundswith the processing oil were made in the same wayand the processing oil was added to the rubber atthe start of mixing. For making the BR compounds,the filler and rubber were mixed together for 10 minand then TBBS, TMTD and ZnO were added andmixed for another 6 min. In total, 45 SBR and BRrubber compounds were made.To make the SBR/BR rubber blends, SBR was

mixed with the processing oil for 2 min and then BRwas added and mixed for 1 min. Silica was subse-quently added and mixed for 3 min after which,TBBS, ZnO, TMTD, and antidegradant were incorpo-rated and mixed for another 6 min before the com-pounds were removed from the mixer. Temperatureof the rubber compounds reached 53�C during mix-ing. Before the curing chemicals were added, therotors were stopped and the rubber compoundswere cooled down to below 50�C to avoid scorchduring the subsequent mixing.Finally, when mixing ended, the rubber com-

pounds were removed from the mixer and milled toa thickness of 6–8 mm for further work. The com-pounds were stored at 21 6 2�C for at least 24 hbefore their viscosity and cure properties weremeasured.

Addition of TBBS to the silanizedsilica-filled SBR and BR rubbers

Accelerators were added to control the onset andrate of cure as well as crosslink density of the rub-bers. To activate the rubber reactive tetrasulfanegroups of TESPT, TBBS was added. The loading ofTBBS in the silica-filled SBR rubber was increasedprogressively to 9 phr to measure the minimumamount needed to optimize Dtorque and the chemi-cal bonding or crosslinking between the rubber andfiller via TESPT. The minimum amount of TBBSmeasured for the silica-filled SBR rubber was subse-quently added to the silica-filled BR rubber. Notethat Dtorque is an indication of crosslink densitychanges in the rubber. The formation of crosslinksstrengthened the rubber/TESPT interaction.4 In total,seven compounds were made (compounds 1–7;Table I).

Addition of zinc oxide to the silanizedsilica-filled SBR and BR rubbers with TBBS

Zinc oxide was added to enhance the effectivenessof TBBS during the curing reaction in the rubbers.The loading of ZnO in the silica-filled SBR and BRrubbers with TBBS was raised to 2.5 phr to deter-mine the minimum amount needed to maximizeDtorque and the efficiency of TBBS and cure. In

Figure 1 SEM photograph showing good dispersion ofthe silica particles in the rubber. Data for the SBR rubberwith mixing time ¼ 10 min.

1520 SAEED ET AL.


total, 18 compounds were mixed (compounds 8–14;Tables II and compounds 15–25; Table III).

Addition of TMTD to the silanized silica-filledSBR and BR rubbers

To activate the rubber reactive tetrasulfane groups ofTESPT, TMTD was added. The loading of TMTD inthe silica-filled SBR rubber was increased progres-sively to 8.5 phr to measure the minimum amountneeded to optimize Dtorque and the chemical bond-ing between the rubber and TESPT. The minimumamount of TMTD measured for the silica-filled SBRrubber was then included in the silica-filled BR rub-ber. In total, six compounds were made (compounds26–31; Table IV).

Addition of zinc oxide to the silanizedsilica-filled SBR and BR rubbers with TMTD

The loading of zinc oxide in the silica-filled SBR andsilica-filled BR rubbers with TMTD was raised to 2.5phr to determine the minimum amount needed tomaximize Dtorque and the efficiency of TMTD andcure. In total, 10 compounds were made (com-pounds 32–41; Table V).

Effect of the processing oil on the cure propertiesof the silica-filled SBR rubber with 5 phr TMTD

To assess effect of the processing oil on the cureproperties of the silanized silica-filled SBR withTMTD, four compounds were made. The loading of

the processing oil was increased from 0 phr to 15phr (compounds 42–45; Table VI).

Mooney viscosity and cure propertiesof the rubber compounds and the blends

The viscosity of the rubber compounds was measuredat 100�C in a single-speed rotational Mooney viscome-ter (Wallace Instruments, Surrey, UK) according to aBritish Standard21 (Table VII). The scorch time, whichis the time for the onset of cure, and the optimumcure time, which is the time for the completion ofcure, were determined from the cure traces generatedat 140 6 2�C and 170 6 2�C by an oscillating discrheometer curemeter (ODR, Monsanto, Swindon, UK)at an angular displacement of 6 3o and a test fre-quency of 1.7 Hz.22 The cure rate index, which is ameasure of the rate of cure in the rubber, was calcu-lated using a British Standard.23 The rheometer testsran for up to 2 h. Figure 2 shows typical cure tracesproduced for the rubber compounds. Dtorque, is thedifference between the maximum and minimum tor-que values on the cure trace of the rubber, and asmentioned earlier, indicates changes in the crosslinkdensity4 and was calculated from these traces.Dtorque was subsequently plotted against the loadingof TBBS, ZnO, and TMTD. Results from these experi-ments are also summarized in Tables I–VIII.

Preparation of test pieces and testing of rubbercompounds

The rubber compounds were cured in a compressionmold with a pressure of 11 MPa. Pieces of rubber,

TABLE IFormulation and ODR Test Results for the Silica-filled SBR Rubber with an

Increasing Loading of TBBS

Compound no

1 2 3 4 5 6 7

TBBS (phr) 0.5 1.5 3.0 4.5 6.0 7.5 9.0Minimum torque (dN m) 26 27 22 21 18 18 17Maximum torque (dN m) 34 43 44 46 43 47 43Dtorque (dN m) 8 16 22 25 25 29 26

Formulation: 100 phr styrene-butadiene rubber and 60 phr silica.

TABLE IIFormulation and ODR Test Results for the Silica-filled SBR Rubber with an

Increasing Loading of Zinc Oxide

Compound no

8 9 10 11 12 13 14

ZnO (phr) 0 0.3 0.5 1.0 1.5 2.0 2.5Minimum torque (dN m) 22 26 24 22 20 18 19Maximum torque (dN m) 44 69 81 85 82 80 83Dtorque (dN m) 22 43 57 63 62 62 64

Formulation: 100 phr styrene-butadiene rubber, 60 phr silica, and 3 phr TBBS.



each approximately 140 g in weight, were cut fromthe milled sheet 6 mm thick. Each piece was placedin the center of the mold to enable it to flow in allthe directions when pressure was applied to preventanisotropy from forming in the cured rubbers.

For determining the tensile properties, tearstrength, Young’s modulus, modulus at differentstrain amplitudes, cyclic fatigue life and tan d of therubbers, sheets 23 cm by 23 cm by � 2.7 mm thickwere cured at 170 6 2�C, from which various sam-ples for further tests were cut. For measuring the ab-rasion resistance and bound rubber content, cylindri-cal samples 16 mm in diameter and 8 mm in heightwere cured at 170 6 2�C. For the hardness determi-nation, cylindrical samples 29 mm in diameter and13 mm in height, and for the heat build-up, samples17 mm in diameter and 25 mm in height were curedat 140 6 2�C. The bigger samples were cured at alower temperature to ensure full cure in the centerof the rubber.

Bound rubber content of the rubber vulcanizates

The solvent used for the swelling tests and boundrubber determination was toluene. For the determi-nation, the samples were placed individually in 100ml of the solvent in labeled bottles and allowed toswell for up to 18 days at ambient temperature (216 2�C). The weight of the samples was measuredevery day until it reached equilibrium. It took up to4 days for the rubber samples to reach equilibrium.

The solvent was removed after this time elapsed,and the samples were dried in air for 9 h. The sam-ples were subsequently dried in an oven at 85�C for24 h and allowed to stand for an extra 24 h at ambi-ent temperature before they were re-weighed. Thebound rubber was then calculated using the follow-ing expression24

RB ¼Wfg �W

mf

ðmfþmpÞh i

Wmp

ðmfþmpÞh i � 100 (1)

where Wfg is the weight of silica and gel, mf theweight of the filler in the compound, mp the weightof the polymer in the compound, and W the weightof the specimen.

Hardness, cohesive tear strength, tensile properties,and cyclic fatigue life of the rubber vulcanizates

The hardness of the rubber vulcanizates was meas-ured in a Shore A Durometer hardness tester (TheShore Instrument and MFG, Co., New York) at am-bient temperature (20�C) over a 15-s interval afterwhich a reading was taken. This was repeated atfour different positions on each sample and the me-dian of the four readings was calculated.25 The teartests were performed at an angle of 180o, at ambienttemperature (20�C) and at a constant cross-headspeed of 50 mm/min.26 For each rubber, five trousertest pieces were fractured and the average tearing

TABLE IVFormulation and ODR Test Results for the Silica-filled SBR Rubber with an

Increasing Loading of TMTD

Compound no

26 27 28 29 30 31

TMTD (phr) 0.5 2.0 3.5 5.0 7.0 8.5Minimum torque (dN m) 29 27 25 24 23 22Maximum torque (dN m) 39 45 67 78 83 89Dtorque (dN m) 10 18 42 54 60 67

Formulation: 100 phr styrene-butadiene rubber, 60 phr silica.

TABLE IIIFormulation and ODR Test Results for the Silica-filled BR Rubber with an Increasing Loading of Zinc Oxide

Compound no

15 16 17 18 19 20 21 22 23 24 25

ZnO (phr) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.8 1 1.5 2.0Minimum torque (dN m) 53 52 49 53 50 61 58 55 51 52 50Maximum torque (dN m) 106 125 129 131 131 138 138 133 140 131 129Dtorque (dN m) 53 73 80 78 81 77 80 78 89 79 79

Formulation: 100 phr polybutadiene rubber, 60 phr silica, and 3 phr TBBS.

1522 SAEED ET AL.


force was used to calculate tearing energy, T, for therubbers.27 The median values of the tearing energieswere subsequently noted.

The tensile stress, elongation at break, and storedenergy density at break of the rubbers were deter-mined in uniaxial tension using standard dumbbelltest pieces. The tests were performed at 20�C and ata cross-head speed of 50 mm/min.28 The median ofthe three values were subsequently noted. LloydNexygen 4.5.1 computer software was used for stor-ing and processing the data.

The cyclic fatigue life of the rubbers (number ofcycles to failure) was measured in uniaxial tension ina Hampden dynamic testing machine (Northampton,UK) with standard dumbbell test pieces. The testpieces were die-stamped from the sheets of cured rub-ber. The tests were performed at a constant maximumdeflection of 100% and a test frequency of 1.4 Hz.29

The test temperature was 21 6 2�C, and the strain oneach test piece was relaxed to zero at the end of eachcycle. For each rubber, eight test pieces were cycled tofailure, and the tests were stopped whenever the num-ber of cycles exceeded 1000 kilocycles (kc).

Modulus at different strain amplitudes,abrasion resistance, and heat build-upof the rubber vulcanizates

The modulus of the vulcanizates at 50, 100, 200, and300% strain amplitudes and Young’s modulus weremeasured in uniaxial tension, using standard dumb-bell test-pieces. The tests were carried out at ambienttemperature (20�C) at a cross-head speed of 50 mm/min.28 Nexygen 4.5.1 computer software was used tostore and process the data. The tear strength, tensileproperties and modulus of the rubber vulcanizateswere measured in a Lloyd testing machine LR50K(Hampshire, UK).The abrasion resistance of the cured rubbers was

measured according to a British Standard30 and wasexpressed as abrasion resistance index (ARI). Anindex value of greater than 100% indicated that thetest compound was more resistance to abrasion thanthe standard rubber under the conditions of the test.The heat build-up of the rubber compounds wasdetermined using cylindrical test pieces and the tem-perature rises on the inside and on the surface of thetest pieces were measured.31

TABLE VFormulation and ODR Test Results for the Silica-filled SBR and BR Rubbers

with an Increasing Loading of Zinc Oxide

Compound no

32 33 34 35 36 37 38 39 40* 41*

ZnO (phr) 0 0.13 0.3 0.5 1.0 1.5 2.0 2.5 0 0.1Minimum torque (dN m) 24 29 31 32 30 28 29 23 51 66Maximum torque (dN m) 78 124 142 150 157 167 167 165 133 197Dtorque (dN m) 54 95 111 118 127 139 138 142 82 131

Formulations: Compounds 32-39: 100 phr styrene-butadiene rubber, 60 phr silica, 5 phr TMTD.*Compound 40: 100 phr polybutadiene, 60 phr silica, 5 phr TMTD, 0 phr ZnO.*Compound 41: 100 phr polybutadiene, 60 phr silica, 5 phr TMTD, 0.1 phr ZnO.

TABLE VIFormulation and ODR Test Results for theSilica-filled SBR Rubber with an Increasing

Loading of Processing Oil

Compound no

42 43 44 45

Enerflex 74 (phr) 0 5 10 15Minimum torque (dN m) 29 27 23 18Maximum torque (dN m) 78 76 58 46Dtorque (dN m) 49 49 35 28Scorch time, ts2 (min) 10 10 14 16Optimum cure time, t95 (min) 61 58 83 94Cure rate index (min�1) 2.0 2.1 1.5 1.3

Formulation: 100 phr styrene-butadiene rubber, 60 phrsilica, 5 phr TMTD.

TABLE VIIMooney Viscosity and Glass-Transition TemperatureValues of the Pure Rubbers, Pure SBR/BR Rubber

Blend, and Compounds 46 and 47

Rubber

Mooneyviscosity(MU)

Glass-transitiontemperature

(�C)

SBR 52 �50BR 49 �107Pure SBR/BR blend 40 –Compound 46 98 �79Compound 47 84 �80

Compound 46: SBR/BR rubber blend cured with TBBSand ZnO.Compound 47: SBR/BR rubber blend cured with TMTD.



Glass transition temperature and loss tangent(tan d) of the raw SBR and BR rubbers andSBR/BR rubber blends

A modulated-temperature differential scanning calo-rimeter (model 2920, TA Instruments, New Castle,DE) was used to measure the glass transition tem-perature, Tg, of the pure BR and SBR rubbers andthe SBR/BR rubber blends. An oscillation amplitudeof 1�C and a period of 60 s were used throughoutthe investigations, which were conducted at a heat-ing rate of 3�C/min. TA Instruments Graphwaresoftware was used to measure the heat flow and

heat capacity. The calorimeter was calibrated withindium standards. Both temperature and baselinewere calibrated as for conventional DSC. A standardaluminum pan and lid were used and samples ofrubber approximately 9–10 mg in weight wereplaced in the pan at ambient temperature and thelid was subsequently closed under some nominalpressure. The assembly was placed in the chamberof the calorimeter and the temperature was loweredto �140�C with the flow of liquid nitrogen. Nitrogengas was also used with a flow rate of 35 ml/min asa medium.Tan d is the ratio between loss modulus and elas-

tic modulus. The loss modulus represents the vis-cous component of modulus and includes all theenergy dissipation processes during dynamic strain.The tan d was measured in DMAQ800 model CFL-50 (TA Instruments, USA), using Universal Analysis2000 Software Version 4.3A. Test pieces 35 mm long,10 mm wide and � 2.70 mm thick were used. Thetests were performed at 1, 10, and 100 Hz frequen-cies. The samples were deflected by 256 lm (nomi-nal peak to peak displacement) during the test, andthe sample temperature was raised from �140 to120�C at 3�C/min steps.

RESULTS AND DISCUSSION

Effect of TBBS and ZnO on the Dtorque of thesilica-filled SBR and BR rubber compounds

Figure 3 shows Dtorque as a function of TBBS load-ing for the silica-filled SBR. Dtorque increased to 22dNm as the loading of TBBS was raised to 3 phr.Further increases to 9 phr had a lesser effect on theDtorque value, which rose to 26 dN m. Evidently, 3phr was sufficient to activate the rubber reactive tet-rasulfane groups of TESPT and form crosslinksbetween the rubber and filler. Totally, 3 phr TBBSwas subsequently added to the silica-filled BR rub-ber compound.

Figure 2 Torque versus time traces by ODR for the SBR/BR rubber blends at 170�C. (---) compound 46, (__) com-pound 47.

TABLE VIIIFormulations and Cure Propertiesof the SBR/BR Rubber Blends

Compound no

46 47

SBR 75 75BR 25 25Silanized silica 60 60TBBS 3 –TMTD – 5ZnO 0.5 –Santoflex 13 1 1Enerflex 74 7.5 7.5ODR results at 170�CMinimum torque (dN m) 20 20Maximum torque (dN m) 72 67Dtorque (dN m) 52 47ts2 (min) 4 2t95 (min) 13 9Cure rate index (min�1) 11 14.3ODR results at 140�CMinimum torque (dN m) 22 24Maximum torque (dN m) 80 90Dtorque (dN m) 58 66ts2 (min) 19 7t95 (min) 65 55Cure rate index (min�1) 2.2 2.1 Figure 3 DTorque versus TBBS loading for the silica-

filled SBR rubber.

1524 SAEED ET AL.


To enhance the crosslink density of the silica-filledSBR rubber compound with 3 phr TBBS, ZnO wasadded. Dtorque increased to 57 dNm when 0.5 phrZnO was incorporated in the rubber and showedonly a slight improvement to 64 dNm thereafterwhen an extra 2 phr ZnO was added (Fig. 4). Simi-larly, for the silica-filled BR compound with 3 phrTBBS, Dtorque increased to 80 dNm with 0.2 phrZnO and showed no further improvement whenanother 1.8 phr ZnO was included in the rubber. AnSBR/BR rubber blend was made by adding 60 phrsilanized silica, 3 phr TBBS, 0.5 phr ZnO, 7.5 phrprocessing oil, and 1 phr antidegradant to the SBRand BR rubbers (compound 46; Table VIII).

Effect of TMTD and ZnO on the Dtorqueof the silanized silica-filled SBR andBR rubber compounds

Figure 5 shows Dtorque against the loading ofTMTD for the silica-filled SBR compound. Dtorqueincreased sharply to 53 dNm when 5 phr TMTDwas added and then, it continued rising at a much

slower rate to 67 dNm when the loading of TMTDreached 8.5 phr.Apparently, 5 phr TMTD was sufficient to activate

the rubber reactive tetrasulfane groups of TESPTand cure the rubber. Totally, 5 phr TMTD was sub-sequently mixed with the silica-filled BR rubbercompound.To improve the efficiency of TMTD, ZnO was

mixed with the silica-filled SBR and BR rubber com-pounds. Dtorque of the silica-filled SBR compoundwith 5 phr TMTD increased rapidly to 111 dNm af-ter 0.3 phr ZnO was added, and it continued risingto 142 dNm as the loading of ZnO was raised to 2.5phr. Similarly, the Dtorque of the silica-filled BRcompound with 5 phr TMTD rose from 82 dNm to131 dNm with 0.1 phr ZnO, and no more ZnO wasadded because it would have made the rubber toobrittle (Fig. 6). An SBR/BR rubber blend was pre-pared by adding 60 phr silanized silica, 5 phrTMTD, 7.5 phr processing oil, and 1 phr antidegra-dant to the SBR and BR rubbers (compound 47;Table VIII).

Effect of the processing oil on the cure propertiesof the silica-filled SBR rubber with 5 phr TMTD

Aromatic oils are often added as processing aid toreduce viscosity and increase processibility of rubbercompounds. Passenger car tire tread compounds cancontain up to 37 phr processing oil.32 Previous stud-ies33 showed that adding up to 10 phr processing oilhad no adverse effect on the scorch and optimumcure times of a sulfur-cured SBR rubber but Dtorquereduced when the loading of the oil was increasedin the rubber.To assess effect of the processing oil on the cure

properties of the silica-filled SBR rubber compoundwith 5 phr TMTD, the loading of the oil was raisedto 15 phr. The scorch and optimum cure timesincreased by approximately 60% and 54%,

Figure 4 DTorque versus ZnO loading. (l), silica-filledSBR rubber with 3 phr TBBS; (^), silica-filled BR rubberwith 3 phr TBBS.

Figure 5 DTorque versus TMTD loading for the silica-filled SBR rubber.

Figure 6 DTorque versus ZnO loading. (l), silica-filledSBR rubber with 5 phr TMTD; (^), silica-filled BR rubberwith 5 phr TMTD.



respectively, and the cure rate index decreased by35%. This was in contrast to what was reported pre-viously.33 However, the addition of the oil did havea detrimental effect on Dtorque, which dropped by43%. This indicated a significant reduction in thecrosslink density of the rubber (Table VI).

Viscosity and cure properties of theSBR/BR rubber blends

To optimize the reinforcing effect of the filler on themechanical properties of the vulcanizates, it wasessential to disperse the filler particles well in therubber.34 The BR and SBR compounds were mixedfor 16 and 10 min, respectively, to achieve good dis-persion of the silica particles. However, long mixingtimes, for example 10 min, breaks down the rubberand causes reduction in its molecular weight35 andviscosity.36 The reduction is due to chain scission,or, the mechanical rupture of the primary carbon–carbon bonds that are present along the backbone ofthe rubber chains.37 This is often compensated bythe reinforcing effect of the filler.

The addition of silica, TBBS, TMTD, ZnO, antide-gradant, and processing oil increased the viscosity ofthe raw SBR/BR rubber blend from 40 MU to 98MU for compound 46 and 84 MU for compound 47(Table VII). Note that both compounds had similarglass transition temperatures around �80�C (TableVII). Fillers increase rubber viscosity because of theformation of bound rubber.24 The bound rubber con-tent of compounds 46 and 47 was 69% (Table IX),and this indicated a high level of rubber-filler inter-action. It is known that bound rubber forms duringmixing while filler dispersion occurs38 and increasesas a function of mixing temperature,39 mixingtime,40 and storage time.41 Since the mixing timewas 12 min for compounds 46 and 47, and the tem-perature rose to 53�C during mixing, bound rubberformed. Furthermore, the rubber blends were storedat ambient temperature (21 6 2�C) for three to fourdays before and one week after they were cured.This also helped to increase the bound rubber,which reinforced the mechanical properties of therubber blends. compound 47 had shorter scorch andoptimum cure times and a faster cure rate (cf, Com-pounds 46 and 47; Table VIII). TMTD is a fast curingaccelerator, which explained the shorter cure cycleof compound 47.

Mechanical properties of the curedSBR/BR rubber blends

The mechanical properties of the SBR/BR rubberblends are summarized in Table IX. With the excep-tion of the hardness and tearing energy, the remain-ing properties of compound 46 were noticeably bet-

ter than those of compound 47. For example, tensilestrength, elongation at break, and stored energy den-sity at break were higher by � 46%, 6% and 45%,respectively. The Young’s modulus was the same forboth compounds at 4 MPa. The modulus of com-pound 46 was 13–25% higher as the strain amplitudeon the rubber was increased from 50 to 300%,respectively, (cf, compound 46 with compound 47;Table IX). The properties which are of significant im-portance to tire tread performance were also supe-rior for compound 46. For instance, the heat build-up tests measured internal and surface temperaturesof 148.7�C and 85.7�C, respectively, for compound46, which were almost 8% lower than those meas-ured for compound 47. Probably, the most interest-ing result was for the abrasion resistance index ofcompound 46. It was 34% higher than that of com-pound 47 and this indicated a much better resistanceto abrasion. The cyclic fatigue life of both com-pounds exceeded 1000 kc.Rubber reinforcement is to a great extent due to

filler–rubber adhesion,4 filler–filler interaction,42 andcrosslink density.11,43 The filler–filler interaction wasnegligible because the silica particles were dispersedwell in the rubbers. The chemical bonding betweenthe rubber and filler was optimized and the boundrubber was 69%. This indicated strong filler–rubberadhesion. The Dtorque of the two compounds were52 and 47 dN m, at 170�C, respectively, (Table VIII),which also indicated contribution from crosslinks to

TABLE IXBound Rubber and Mechanical Properties

of the SBR/BR Rubber Blends

Compound no

46 47

Swelling tests dataBound rubber content (%) 69 69

Mechanical propertiesHardness (Shore A) 66 69Young’s modulus (MPa) 4 4

Modulus at different strain amplitudes (MPa)Strain amplitude (%)

50 1.6 1.4100 1.7 1.4200 1.9 1.4300 2.0 1.5Tensile strength (MPa) 26 14Elongation at break (%) 1030 968Stored energy density at break(MJ/m3)

121 67

T (kJ/m2) 36.5 86Range of values 32–38 58–103Cyclic fatigue life at 1.4 Hz (kc) >1000 >1000

Heat buildup and abrasion resistance indexInternal temperature (�C) 148.7 161Surface temperature (�C) 85.7 92.3Abrasion resistance index (%) 230 151

1526 SAEED ET AL.


the rubber properties. The surfaces of silica hadbeen pretreated with rubber reactive TESPT, and thesurface area of the filler particles was less than 400m2/g, which also contributed to the improvement inthe mechanical properties of the rubber blends. Inaddition, when the SBR and BR rubbers were mixedtogether for 12 min to produce Compounds 46 and47 and then cured at 170�C for up to 13 min, astrong interphase was formed between the two rub-bers.44,45 The development of a strong interphasebetween dissimilar rubbers is an important factor inthe durability and performance of rubber blends inservice.

Tan d of the cured SBR/BR rubber blendsat different test frequencies

The energy loss in car tires during dynamic strainaffects their service performance such as rollingresistance.38 Rolling resistance is related to themovement of the whole tire corresponding to defor-mation at a frequency 10–100 Hz and a temperatureranging from 50 to 80�C. To meet the requirements

of high-performance tires, a low tan d value at atemperature of 50–80�C to reduce rolling resistanceand save energy and fuel is often required. A hightan d value (high hysteresis) at low temperatures forexample, �50 to �30�C, to obtain high-skid resist-ance and ice and wet-grip is also essential.38

Figures 7-9 show tan d as a function of the testtemperature at 1, 10, and 100 Hz test frequencies,respectively, for compounds 46 and 47. The peak tand values were also summarized in Table X. At 1 Hz,the peak tan d of compounds 46 and 47 were 0.47 at�36.6�C and 0.49 at �35.5�C, respectively, (Fig. 7).At 10 Hz, the peak tan d of compound 46 increasedto 0.53 at �31.7�C and that of compound 47 to 0.50at �30.7�C, respectively, (Fig. 8). At 100 Hz, thepeak tan d for compounds 46 and 47 were 0.72 at�24.0�C and 0.55 at -32.4�C, respectively, (Fig. 9).Clearly, compound 46 possessed a better skid resist-ance and ice and wet-grip at 10 and 100 Hz becauseit had higher tan d values at low temperatures.However, at above 50�C, which is important toreduce the rolling resistance and save energy, thetan d of both compounds were fairly similar at 1 Hzbut that of compound 47 was slightly lower at 10Hz. Notably, increases in the test frequency had asignificant effect on the peak tan d of these

Figure 7 Tan d versus the temperature at 1 Hz. (---) com-pound 46, (__) compound 47.

Figure 8 Tan d versus the temperature at 10 Hz. (----)compound 46, (__) compound 47.

Figure 9 Tan d versus the temperature at 100 Hz. (---)compound 46, (__) compound 47.

TABLE XPeak Tan d and Test Temperature Data for the two SBR/BR Rubber Blends Tested at Different Test Frequencies

Compound

46 1 Hz 47Peak tan d 0.47 0.49Temperature (�C) �36.6 �35.5

10 HzPeak tan d 0.53 0.50Temperature (�C) �31.7 �30.7

100 HzPeak tan d 0.72 0.55Temperature (oC) �24.0 �32.4



compounds. For example, the peak tan d of com-pound 46 rose by more than 50% and that of com-pound 47 by � 12% when the test frequency wasincreased to 100 Hz (Figs. 10 and 11, respectively). Itwas concluded that compound 46, which also had amuch higher abrasion resistance index and a lowerheat build-up, could potentially replace the passen-ger car tire treads currently in use.

Health, safety, and the environmental issuesrelated to passenger car tires

Excessive use of the curing chemicals is harmful tohealth, safety and the environment and their use isrestricted by the new European chemicals policy,Registration, Evaluation, Authorization and Restric-tion of Chemicals (REACH) and various legislationsfor environment and safety. The presence of ZnO intread compounds has also come under growingscrutiny because of environmental concerns.46 Thetire industry is the largest single market for ZnO.The elimination or a more restricted use of ZnO inrubber compounds will help to reduce damage tothe environment.

The average weight of a new radial passenger cartire is � 12.5 kg.47 The tread weight is 32.6% of thetotal weight of a tire and is roughly 4.1 kg. As men-tioned earlier,7 formulations for passenger car tiretread compound can contain up to 11 phr chemicalcuratives that is 4.8% of the total weight of all theingredients in the formulation. On this basis, thetotal amount of the chemical curatives in a tire treadcompound can be as high as 197 g. As shown in Ta-ble VIII, compound 46 was fully cured with 3 phrTBBS and 0.5 phr ZnO. The chemical curatives inthis compound were 2% of the total weight of all theingredients in the formulation. If this compoundwas to replace the current tire tread compounds, itwould reduce the loading of the chemical curativesby � 58% to 82 g. Moreover, a large reduction in the

use of ZnO will be greatly beneficial to tire manufac-turers and will ultimately reduce costs and minimizedamage to the environment.

CONCLUSIONS

From this study, it was concluded that:

1. The SBR/BR blend which was cured with thenon-sulfur donor accelerator and zinc oxide hadsuperior tensile strength, elongation at break,stored energy density at break and modulus atdifferent strain amplitudes. It also possessed alower heat build-up, a higher abrasion resistance,a higher tan d value at low temperatures toobtain high skid resistance and ice- and wet-grip.

2. Optimizing the chemical bonding between therubber and filler via the tetrasulfane groups ofTESPT reduced excessive use of the chemicalcuratives significantly. This improved healthand safety at work place and reduced damageto the environment.

3. The addition of the processing oil had a detri-mental effect on the scorch and optimum curetimes, rate of cure and Dtorque of the silanizedsilica-filled SBR rubber cured with TMTD.

The authors thank Evonik Industries AG of Germany forsupplying the silica filler. The scanning electron microscopyof the samples was carried out at the Loughborough Materi-als Characterization Centre, UK. The abrasion resistanceindex and heat build-up measurements were carried out atthe Korea Institute of Footwear and Leather Technology inBusan, Korea.

References

1. Dannenberg, E. M. Rubber Chem Technol 1952, 25, 843.2. Hamed, G. R. Rubber Chem Technol 2000, 73, 524.3. Wolff, S. Rubber Chem Technol 1982, 55, 967.

Figure 10 Tan d versus the temperature at 1, 10, and 100Hz for compound 46.

Figure 11 Tan d versus the temperature at 1, 10, and 100Hz for compound 47.

1528 SAEED ET AL.


4. Wolff, S. Rubber Chem Technol 1996, 69, 325.5. Song, M.; Wong, C. W.; Jin, J.; Ansarifar, A.; Zhang, Z. Y.;

Richardson, M. Polym Int 2005, 54, 560.6. Wagner, M. P. Rubber World August 1971, 46.7. Wang, M. J.; Kutsovsky, Y.; Zhang, P.; Murphy, L. J.; Laube,

S.; Mahmud, K. Paper no. 55, presented at a meeting of theRubber Division, ACS, Providence, RI, April 2001, 24.

8. Dannenberg, E. M. Rubber Chem Technol 1975, 48, 410.9. Ostad-Movahed, S.; Ansar Yasin, K.; Ansarifar, A.; Song, M.;

Hameed, S. J Appl Polym Sci 2008, 109, 869.10. Salgueiro, W.; Marzocca, A.; Somoza, A.; Consolati, G.; Cer-

veny, S.; Quasso, F.; Goyanes, S. Polymer 2004, 45, 6037.11. Kok, C. M.; Yee, V. H. Eur Polym Mater 1986, 22, 341.12. Hair, M. L.; Hertl, W. J Phys Chem 1970, 74, 91.13. Hockley, J. A.; Pethica, B. A. Trans Faraday Soc 1961, 57, 2247.14. Wolff, S.; Gorl, U.; Wang, M. J.; Wolff, W. Eur Rubber J 1994, 16, 16.15. Tan, E. H.; Wolff, S.; Haddeman, M.; Grewatta, H. P.; Wang,

M. J Rubber Chem Technol 1993, 66, 594.16. Rajeev, R. S.; De, S. K. Rubber Chem Technol 2002, 75, 475.17. Ansarifar, A.; Azhar, A.; Ibrahim, N.; Shiah, S. F.; Lawton, J.

M. D. Int J Adhes Adhes 2005, 25, 77.18. Ansarifar, A.; Wang, L.; Ellis, R. J.; Kirtley, S. P.; Riyazuddin,

N. J Appl Polym Sci 2007, 105, 322.19. Ansarifar, A.; Wang, L.; Ellis, R. J.; Haile-Meskel, Y. J Appl

Polym Sci 2007, 106, 1135.20. Ansar Yasin, K.; Ansarifar, A.; Hameed, S.; Wang, L. Polym

Adv 2011, 22, 215.21. Br Standards Institution. Methods of testing raw rubber and

unvulcanized compounded rubber: Methods of physical test-ing. Br Standard 1673: London, UK 1969; Part 3.

22. Br Standards Institution. Methods of test for raw rubber andunvulcanized compounded rubber: Measurement of pre-vul-canizing and curing characteristics by means of curemeter. BrStandard 1673: London, UK 1977; Part 10.

23. Br Standards Institution. Methods of test for raw rubber andunvulcanized compounded rubber: Measurement of pre-vul-canizing and curing characteristics by means of curemeter. BrStandard 903: London, UK 1996; Part A60, Section 60.1.

24. Wolff, S.; Wang, M. J.; Tan, E. H. Rubber Chem Technol 1993,66, 163.

25. Br Standards Institution. Physical testing of rubber: Methodfor determination of hardness. Br Standard 903: London, UK1995; Part A26.

26. Br Standards Institution. Physical testing of rubber: Methodfor determination of tear strength trousers, angle and crescenttest pieces. Br Standard 903: London, UK 1995; Part A3.

27. Greensmith, H. V.; Thomas, A. G. J Polym Sci 1955, 43,189.

28. Br Standards Institution. Physical testing of rubber: Methodfor determination of tensile stress strain properties. Br Stand-ard 903: London, UK 1995; Part A2.

29. Br Standard Institution. Methods of testing vulcanized rubber:determination of resistance to tension fatigue. Br Standard903: London, U. K.; 1986; Part A51.

30. Br Standards Institution. Rubber, vulcanized or thermoplas-tic—Determination of abrasion resistance using a rotating cy-lindrical drum device. ISO 4649: 2002.

31. Br Standards Institution. Methods of testing vulcanized rub-ber. Determination of temperature rise and resistance to fa-tigue in flexometer testing (compression flexometer). BrStandard 903: London, UK 1984, Part A50.

32. Ahmad, S.; Schaefer, R. J (to B. F. Goodrich), US.4, 519, 430(May 28 1985).

33. Nandanan, V.; Joseph, R.; George, K. E. J Appl Polym Sci1999, 72, 487.

34. Cochet, P.; Barruel, P.; Barriquand L.; Grobert, J.; Bomal, Y.;Prat, E.; Poulenc, E. P. Rubber World June 1994, 219, 20.

35. Pike, M.; Watson, W. F. J Polym Sci 1952, 9, 229.36. Harmon, D. J.; Jacobs, H. L. J Appl Polym Sci 1966, 10, 253.37. Ahagon, A. Rubber Chem Technol 1996, 69, 742.38. Wang, M.-J. Rubber Chem Technol 1998, 71, 520.39. Stickney, P. B.; McSweeney, E. E.; Mueller, W. J Rubber Chem

Technol 1958, 31, 31.40. Boonstra, B. B. J Appl Polym Sci 1967, 11, 389.41. Leblanc, L.; Hardy, P. Kautsch Gummi Kunstst 1991, 44,

1119.42. Frohlich, J.; Niedermeier, W.; Luginsland, H. D. Compos A

2005, 36, 449.43. Nasir, M.; Teh, G. K. Eur Polym Mater 1988, 24, 733.44. Ostad-Movahed, S.; Ansarifar, A.; Song, M. J Appl Polym Sci

2009, 111, 1644.45. Ostad-Movahed, S.; Ansarifar, A.; Song, M. J Appl Polym Sci

2009, 113, 1868.46. Walter, J. Tire Technol 2009, 18.47. Rubber Manufacturers Association. Available at: http://rma.

org/scrap_tires/scrap_tires_markets/scrap_tire_charateristics/



Documents

Two advanced styrene-butadiene/polybutadiene rubber blends filled with a silanized silica nanofiller for potential use in passenger car tire tread compound